Abstract

Coal based thermal power plants have been the backbone of a country due to its major contribution in electricity generation for the developmental purposes. Due to indomitable rise in demand of the electricity, the increase in generation of fly ash has become inevitable now-a-days. Moreover, the disposal of fly ash has become a major issue for coal based thermal power plants as it requires a vast disposal area and gives rise to a lot of problems like shortage of useful land, increase in disposal cost and dusting of atmospheric air. Fly ash generated from the coal fired thermal power plants is mostly sluiced into ash ponds by wet disposal method. This fly ash contains a number of soluble major and trace elements such as As, Fe, Cd, Hg, Zn, Pb and Cu, etc. There is possibility that leachate emanating from this fly ash bed may contaminate the ground and pose a threat to the human as well as aquatic life. Therefore, stabilization of fly ash by some chemical additives like lime/cement is an excellent method to mitigate the leachate characteristics of fly ash in addition to improve the strength and stability of the structure.
The leaching of metals mainly depends on two factor such as pH and hydraulic conductivity. The pH plays a pivotal role in reducing the concentration of the elements. Hydraulic conductivity also has a major effect in preventing the leachate from contaminating the ground water. If the material can be made less permeable, the leachate can be confined at the source of generation and thus the ground water can be protected from being contaminated.
A thorough study of the previous research works reveals that lime stabilization is an effective means of reducing the permeability and concentration of metals in the leachate emanating from ash ponds. In the present research work an effort has been taken to study the effect of lime on hydraulic conductivity and leaching characteristics of fly ash by varying the mix-proportion and curing period.
In addition to this, an experimental set up has been developed to investigate the efficacy of lime column in mitigating the leachate characteristics of compacted and sedimented ash beds.
The experiments were performed in two phases. At first, the compaction characteristics of fly ash mixed with different lime content such as 0%, 2%, 4%, 8%, 12% and 15% were found out from light and heavy compaction tests. The hydraulic conductivity and leachate characteristics of compacted fly ash-lime mixes were determined after 0, 7, 15, 30, 60 and 90 days of curing. All these samples were prepared corresponding to their respective MDD and OMC and cured for the specified curing periods as mentioned above. The concentration of the major and trace elements like Cu, Fe, Zn, Ca, Ni, Pb and Cr were found out by atomic absorption spectrometer. The effects of lime content and curing period on microstructure, morphology and hydration products in the stabilized specimens were studied by various microanalyses such as XRD and SEM tests.
Further, large scale laboratory models of sediment and compacted fly ash beds were prepared with a centrally installed lime column simulating a field condition as closely as possible. The samples were collected from various radial distances and depths after 7, 30, 90 and 180 days of curing period and subjected to different tests such as pH, and leachate analysis of different elements like Ca, Ni, Pb, Zn, Cu, Cr, and Fe. In addition to this, the hydraulic conductivity of treated ash deposit was measured by collecting undisturbed specimens from different radial distances and depths.
From compaction test results it is found that for light compaction test, with increase in lime content the OMC value increases up to 4% and thereafter, it decreases whereas in case of heavy compaction test, the OMC increases up to 2% lime addition and thereafter, it decreases. Similarly, the MDD in case of light compaction test decreases with increase in lime content up to 4% and thereafter it increases whereas in case of heavy compaction test the same value decreases up to 2% lime addition and thereafter, it increases.
The hydraulic conductivity value is found to depend on the lime content, compaction effort, and curing period. The samples containing higher doses of lime shows significant decrease in hydraulic conductivity value. It was found that at 90 days curing, it reduces about 10 times for samples compacted with light compaction energy whereas in case of heavy compaction, it decreases about 100 times than the unstabilized specimen. However, sample with no lime content showed marginal change in hydraulic conductivity value with curing period. XRD analysis shows the presence of compounds like ettringite, C-S-H and C-A-H gel which blocks the pore space and reduces the capillary voids. SEM analysis also confirms an interlocking network of hydration products which is responsible in reducing the hydraulic conductivity.
From the leachate analysis, it was observed that concentration of all elements was less than that of leachate sample of raw fly ash collected from acid digestion and extraction method. At 0 days curing the concentration of each metal (except Ca) is approximately same for different lime contents whereas at higher curing period, as the lime content increases, the concentration of metals in the leachate follows a decreasing trend. This is due to presence of alkaline medium which is unfavorable for metal precipitation and also due to encapsulation of metals by the hydration products. It is also observed that the concentration of all the metals was below the threshold limit of IS-10500 and WHO water quality standard.
The pH test results of the sample collected from sediment and compacted ash deposits show that the value is more in the sample collected adjacent to the lime column than that of the sample collected at a remote area from the lime column and also it increases with increase in depth. This is due to migration of lime to the periphery and downward direction of the tank. Moreover, it is observed that the pH value increases with curing period up to 180 days and thereafter, it decreases. Because with longer curing period lime is consumed in pozzolanic reaction which results in reduction of the pH value.
The permeability test result of lime column stabilized ash bed shows that during the early period of stabilization (30 days of curing) no significant variation in hydraulic conductivity value is observed in specimens collected at different locations (different radial distances and depths). However, as the curing period increases, the hydraulic conductivity follows an increasing trend with increase in radial distance whereas the same decreases with depth. In addition, it is also observed that as the curing period increases, a significant reduction in hydraulic conductivity occurs in all the layers of sedimented pond ash deposit.
The leachate analysis result shows that concentration of elements in the leachate sample collected from the test tank is much lower than the leachate sample extracted from raw fly ash. At early stage of curing the concentration of Ca is found to be more than that in the virgin fly ash, however at longer curing period, i.e at 365 days, the concentration of Ca is found to be decreased due to participation of lime in pozzolanic reaction. It is also observed from the results that the concentration of major and trace elements in the leachate sample collected adjacent to the lime column is lesser than that of the sample collected at the periphery of the test tank. This is due to higher pH value adjacent to the lime column as compared to remote areas. Moreover, the concentration of other elements in the leachate collected on 365 days curing is less than that of the sample collected on 90 and 180 days. This is due to the formation of hydration product such as C-S-H gel which encapsulates the elements and prevents leaching. The concentration of elements is found to be less than the threshold limit of WHO and IS-10500 water quality standard.Thus, it is concluded that lime treatment is an effective means of reducing the hydraulic conductivity and concentration of metals in the leachate emanating from compacted as well as sedimented fly ash specimens.